Question

At 25oC, Kp = 5.3 × 105 for the reaction
N2(g) + 3H2(g) ⇌ 2NH3(g)
When a certain partial pressure of NH3(g) is put into an otherwise empty rigid vessel at 25oC, equilibrium is reached when 50.0% of the original ammonia has decomposed. What was the original partial pressure of ammonia before any decomposition occurred?